Go to a tux shop or men's clothing department of Macy's or some such and get measured properly.

Write it down.

Then, find a good rental company. I've had nothing but stellar service from this vendor:

http://scotyard.com

And I'm not paid to say it. We've rented large groups of complete kilt outfits from them, and have not one complaint or reservation.

Mostly, though, have fun! Life's too short not to.

Oh, I should mention...I'm a 48 short - an impossible size to find. They were terrific.